System and method for generating a discharge in high pressure gases

ABSTRACT

A method of generating an electrical discharge in a high pressure gas contained in a sealed enclosure. The method includes driving a helical coil resonator at an RF frequency to generate an RF electric-magnetic field sufficient to generate an electrical discharge in the high pressure gas. The electrical discharge produces an emission spectrum that may be spectroscopically analyzed to determine the composition and impurity content of the gas.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/279,684.

FIELD OF THE INVENTION

[0002] The present invention relates to gas composition analysis. Moreparticularly the present invention relates to nondestructive analysis ofhigh pressure gas contained in dielectric enclosures by emissionspectroscopy.

BACKGROUND OF THE INVENTION

[0003] Metal halide and other high intensity discharge (HID) lamps havefound widespread acceptance for lighting large area indoor and outdoorspaces. In the manufacture of HID lamps, it is often desirable toprovide a controlled atmosphere for many of the components of the lampto prevent premature failure of the components and thereby prolong theoperating life of the lamp. For example, the exposure of the arc tube ofan HID lamp to small amounts of oxygen during lamp operation willsignificantly degrade the components leading to lamp failure, thusshortening the operating life of the lamp. Further by way of example,the exposure of the arc tube to hydrogen may lead to diffusion ofhydrogen into the arc tube leading to high starting and re-ignitionvoltages, and ultimately reduced life expectancy of the lamp. To preventthe exposure of such components to damaging atmospheres, it is wellknown to provide a controlled atmosphere for the components byenveloping the components in a desired atmosphere contained within anouter lamp jacket. Typically, the outer jacket of an HID lamp is filledwith an inert gas such as nitrogen.

[0004] In view of the deleterious effects of the presence of impurities,it is desirable to nondestructively analyze the composition and impuritycontent of the gaseous atmosphere contained within the lamp outerjackets. Gas analysis by emission spectroscopy is well known inanalyzing the composition and impurity content of gaseous atmospheres atlow pressures (<about 0.1 atm). However, the gaseous atmospherecontained within an outer jacket of an HID lamp is typically atrelatively high pressure (about 0.1-2.0 atm). There remains a need fornondestructive gas analysis by emission spectroscopy in high pressuregaseous atmospheres.

[0005] Accordingly, it is an object of the present invention to obviatethe deficiencies of the prior art and to provide a novel system andmethod for nondestructive high pressure gas analysis.

[0006] It is another object of the present invention to provide a novelsystem and method for generating a discharge in a high pressure gas.

[0007] It is a further object of the present invention to provide anovel system and method for creating a stable electrical discharge in ahigh pressure gas contained in a sealed enclosure, so that thecomposition and impurity content of the gas can be spectroscopicallyanalyzed without destroying the enclosure.

[0008] It is yet another object of the present invention to provide anovel system and method for emission spectroscopy of gaseousatmospheres.

[0009] It is yet another object of the present invention to provide anovel system and method for nondestructive analysis of HID lamps.

[0010] These and many other objects and advantages of the presentinvention will be readily apparent to one skilled in the art to whichthe invention pertains from a perusal of the claims, the appendeddrawings, and the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic diagram illustrating an RF discharge sourceaccording to an exemplary embodiment of the present invention.

[0012]FIGS. 2 and 3 are schematic diagrams illustrating an exemplaryhelical coil resonator.

[0013]FIG. 4 is a graph illustrating the impedance of a helical coilresonator as a function of frequency at the driving point.

[0014]FIG. 5 is a graph illustrating a spectrum from a discharge in pureN₂ gas at 0.3 atm (300 torr).

[0015]FIG. 6A is a graph illustrating a typical emission spectrum from ahelical coil resonator RF discharge in N₂ gas and 1 percent hydrogen at0.3 atm (300 torr).

[0016]FIG. 6B is a graph illustrating a typical emission spectrum from ahelical coil resonator RF discharge in N₂ gas and 1 percent oxygen at0.3 atm (300 torr).

DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention generally finds utility in generating adischarge in high pressure gas. By way of example only, certain aspectsof the invention will be described in connection with emissionspectroscopy for nondestructive analysis of the gaseous content in HIDlamps.

[0018] According to one aspect, the present invention provides a high Q,single frequency RF discharge source for generating a small localizedand stable electrical discharge (plasma) in a high-pressure (0.1 atm to2 atm) gas contained within the outer jacket of an HID lamp. Thedischarge source includes a helical coil resonator (HCR) for providingsufficient RF energy to generate the discharge. The optical emissionspectrum from the plasma can then be spectroscopically analyzed todetermine the composition of the gas and the gaseous impurity contentwithin the enclosure. Impurity concentrations less than about 0.01percent by volume may be detected. Typically the gaseous atmospherecomprises N₂, and may include gaseous impurities such as O₂, H₂, CO₂,CO, H₂O, CH₄, and the like, which contain elements such as O, H, C,and/or any combinations thereof. The RF discharge source of the presentinvention is capable of establishing and maintaining a discharge in highpressure gas, and consumes very little power, thus being useful in bothlab and production line applications.

[0019] The RF discharge source creates an electrical discharge insidethe enclosure by generating an RF electric-magnetic (electric) fieldthat penetrates through the dielectric wall of the enclosure. Theelectric field required for the discharge is proportional to E/N, whereE is the electric field strength and N the number density of the gas.The gas pressure inside the outer jacket of a HID lamp is typicallyabout 0.5 atm at room temperature, requiring a field strength of about 7kV/cm to establish a discharge. When a discharge is established, theheat generated by the discharge reduces the gas number density (N) thusrequiring less electric field strength (E) to maintain the discharge.

[0020] The optical emission spectrum of the plasma, i.e., the atomic andmolecular emission of species in the excited plasma, is analyzed usingconventional spectroscopic techniques to determine the composition andimpurity content of the high pressure gas in the enclosure. Thistypically involves recording the optical emission spectrum of the plasmaat UV, visible, and near-IR wavelengths at sufficiently high resolutionto resolve the atomic lines of the impurities of interest. The spectrumis analyzed by visual/graphical and/or computer aided data manipulationto measure the magnitude of the spectral peaks of interest. This data iscompared to similar data collected from known standards of theimpurities of interest. The concentrations of the impurities are thencalculated by comparison to the known standards.

[0021]FIG. 1 schematically illustrates the RF discharge source 10according to an exemplary embodiment of the present invention. The RFdischarge source 10 is capable of producing the several thousand voltsneeded to strike a discharge in a high pressure gas contained in asealed enclosure. The RF discharge source 10 comprises an RF powergenerator 12, an impedance-matching network 20, and an HCR 22. The RFpower generator 12 includes an RF signal generator 14, an RF poweramplifier 16, and power meter 18. The RF power generator 12 typicallyoutputs about a few hundred volts and drives the HCR 22 at RFfrequencies from about 100 kHz to greater than 100 MHz, and typicallyabout 10 MHz. The HCR 22 operates like a combination of anopen-circuited quarter-wave transmission line in parallel with aninductor back to ground to step up the voltage from the RF poweramplifier 16 by factors of 20 to 100 or greater.

[0022] The HCR 22 typically includes a wire helix 24 and an electricallyconductive shield 34. As illustrated in FIG. 2, the wire helix 24 isformed by a conductive spiral coil 26 having a first end 28 connected toground and a second opposing end 30 serving as an electrode. An inputtap 32 is located on the coil 26 between the ground (the first end ofthe coil 28) and a point close to the ground.

[0023] Referring again to FIG. 1, the enclosure or outer jacket 40 to beanalyzed is placed in contact with the electrode 30 where the RF voltageis highest. The RF electric field penetrates through the dielectric wallof the enclosure 40 to generate a very stable discharge inside theenclosure 40. A discharge within the outer jacket of an HID lamp may begenerated without exciting the gaseous contents of the arc tube (notshown), which typically contains a low pressure gas such as Ar and a lowpressure vapor such as Hg. The high voltage at the electrode 30 is aresult of the RF power injected at a tap close to the ground point. Inone embodiment, a fiber optic light gathering device 36 collects theoptical emission spectrum of the plasma in the UV, visible, and near IRwavelength ranges, for analysis.

[0024] As discussed above, the operation of the HCR 22 is similar to atransformer, with the voltage being stepped up by the turns ratio of thecoil 26. However, the operation of the HCR 22 is frequency dependent,and its operation may best be modeled by a transmission line cavityhaving a wire length L in the coil (illustrated in FIG. 2 measured fromthe tap point or first end 28 of the coil 26 to the electrode or secondend 30 of the coil 26) being slightly less than one quarter of an RFwavelength. In one embodiment of the present invention, a coil length Lof about 5.5 meters is suitable for an operating frequency of about 13.6MHz (which is set to operate within the allowable FCC bandwidth). Itshould be understood that the coil length and operating frequency arenot limited to these values.

[0025] The electrically conductive shield 34 is typically formed frommetal to enclose the coil 26 as illustrated in FIG. 3 and provides thereturn path for the RF current. The combination of the RF shield 34,coil, and the selection of the tap point 32 enable the HCR 22 to exhibita Q of between about 500 and about 900.

[0026] Referring again to FIG. 2, the input signal is connected to theinput tap 32 between the ground and a point close to the ground. Sincethe total coil length L is about one quarter of the wavelength of the RFelectric field, the highest electric field strength is located at theelectrode 30 formed by the other end of the coil 26. The electric fieldstrength may be raised or lowered by raising or lowering the electricfield strength at the input tap 32.

[0027] The impedance-matching network 20 matches the input impedance ofthe HCR 22 to the output impedance of the RF power generator 12 at thefrequency of operation. In one embodiment, the matching impedance may beabout 50 ohms. At RF frequencies, the input impedance contains bothresistance and reactance. The matching network 20, made up of inductanceand capacitance, may be designed to modify the HCR input impedance to beabout 50 ohms. Moreover, the location of the input tapping point 32should also be considered in matching the input impedance of the HCR 22to the output impedance of the RF power generator 12.

[0028] Specifically, the open-circuit coil can be viewed as anopen-circuit transmission line that is about one quarter wavelength inlength. Since the coil 26 is open circuited, the driving point impedanceZ_(d) of the open part of the circuit is approximately:

Y _(d) =+j Y ₀ tan β1, Z _(d) =−j Z ₀ cot β1

[0029] where Y₀=1/Z₀, Z₀=characteristic impedance of the helicaltransmission line, 1 is the length from the driving point to the tip ofthe electrode, and β is the phase constant. The impedance as a functionof frequency at the driving point is measured and plotted in FIG. 4.This Z_(d.) contains both the real part and imaginary parts. In order tomatch the output impedance of the RF source (50 ohms), aproperly-designed impedance matching network that consists of capacitorsand inductors is inserted between the RF source and the driving point ofthe HCR to obtain the maximum delivery of power of the RF source to theHCR.

[0030] The RF discharge source 10 may be operated by first adjusting thefrequency of the RF signal 14 generator to match the resonant frequencyof the HCR 22. Alternatively the tap point, coil spacing or otherdimensions of the HCR 22 can be adjusted to match the (fixed) frequencyof the RF signal generator 14. In one embodiment, the combination of theRF signal generator 14 and RF power amplifier 16 of the RF powergenerator 12 produces a sinusoidal voltage of about 300 Volts (rms). Thepower generated at the output of the amplifier 16 passes through thepower meter 18, which is capable of measuring both forward and reflectedwave powers.

[0031] The power subsequently goes through the matching network 20before it couples to the HCR 22. The matching network 20 includes avariable capacitor (not shown) which allows the matching impedance ofthe matching network 20 to be selectively adjusted by tuning thecapacitor in order for the electrode 30 of the coil 26 to reach itshighest voltage. The impedance matching network 20 is adjusted tominimize reflected power and maximize “forward” power into the plasmaload and to maximize the physical and temporal stability of the plasma.

[0032] An electric field pick-up device 38 may be provided to monitorthe electrode voltage. The electric field pick-up device 38 includes ametal plate soldered to the center conductor of a coaxial connectorpositioned near the electrode 30. In one embodiment, the signal from thecapacitance pickup may be used to maximize the voltage at the electrode30 as the matching network components are changed.

[0033] It may be necessary in some instances to reduce the heatingeffect of the RF discharge on the dielectric surface of the enclosurecontaining the gas to be analyzed. The RF signal generator 14 mayinclude a gate feature that allows an RF waveform to be duty-cyclemodulated. A suitable modulation frequency may be between 10 and 1000Hz, and typically about 120 Hz, and a duty cycle between 1 percent and99 percent, and typically about 10 percent. The pulsed RF dischargereduces the heating effect of the discharge on dielectric enclosure. Inaddition to reducing the duty cycle of the continuous RF waveform, thegated RF allows analysis of optical emission from excited atoms ormolecules which persist and radiate in the afterglow during the periodwhen the RF source is gated to the “off” state.

[0034] In embodiments of the present invention for generating adischarge in gases at pressures over about 300 torr, a Tesla coil (notshown) has been found to be suitable for initiating the discharge.

EXPERIMENTAL RESULTS

[0035] Emission spectra were recorded using an Acton SpectraPro 300ispectrophotometer over a range of 350 nm to 900 nm with a resolution of0.4 nm FIG. 5 illustrates a spectrum from a discharge in pure N₂ gas at300 torr. FIG. 6A illustrates the analytically useful lines of atomichydrogen (656 nm) and FIG. 6B illustrates the analytically useful linesof atomic oxygen (777 nm) from standards of known 1% hydrogen and 1%oxygen in nitrogen at 300 torr. These atomic lines can be seen asrelatively sharp peaks superimposed on the complex nitrogen molecularband spectra. Detection limits for oxygen and hydrogen in a fillcomprising nitrogen at 500 torr include about 0.3% and 0.1% by volumerespectively.

[0036] While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and the scope of the invention is to be defined solelyby the appended claims when accorded a full range of equivalence, manyvariations and modifications naturally occurring to those of skill inthe art from a perusal hereof.

What is claimed is:
 1. A method of generating an electrical discharge ina high pressure gas contained in a sealed enclosure, the methodcomprising the steps of: contacting a helical coil resonator with adielectric wall of the enclosure; and driving the helical coil resonatorat an RF frequency to generate an RF electric-magnetic field to therebygenerate an electrical discharge in the high pressure gas.
 2. The methodaccording to claim 1, wherein the pressure of the gas is between about0.1 atm and about 2.0 atm.
 3. The method of claim 1, wherein thepressure of the gas is about 0.5 atm.
 4. The method of claim 1, whereinthe RF frequency is about 10 MHz.
 5. The method of claim 1, wherein theRF frequency is between about 10 MHz and about 100 MHz.
 6. The method ofclaim 1, wherein the RF frequency is greater than about 100 MHz.
 7. Themethod of claim 1, wherein the RF frequency is between about 100 kHz andabout 10 MHz.
 8. The method of claim 1, wherein the RF electric-magneticfield generated in the driving step nondestructively penetrates thedielectric wall of the enclosure.
 9. The method of claim 1, wherein thedriving step includes pulsing the RF electric-magnetic field tonondestructively penetrate the dielectric wall of the enclosure.
 10. Themethod of claim 9, wherein the pulsing of the RF electric-magnetic fieldis performed at a frequency of between about 10 Hz and about 1000 Hz.11. The method of claim 10, wherein the pulsing of the RFelectric-magnetic field is performed at a frequency of about 120 Hz. 12.The method of claim 9, wherein the pulsing of the RF electric-magneticfield has a duty-cycle between about 5 percent and about 20 percent. 13.The method of claim 12, wherein the pulsing of the RF electric-magneticfield has a duty-cycle of about 10 percent.
 14. The method of claim 9,wherein the pulsing of the RF electric-magnetic field has a duty-cyclebetween about 1 percent and about 99 percent.
 15. The method of claim 1,wherein the enclosure further contains an arc-tube with a low-pressuregas and a low pressure vapor, the operating step being performed withoutexciting the low pressure gas and the low pressure vapor containedwithin the arc-tube.
 16. The method of claim 1, further comprising thestep of confining the electrical discharge to a narrow flame-likeplasma.
 17. The method of claim 1, wherein the high pressure gas isprimarily molecular nitrogen.
 18. The method of claim 1 furthercomprising the step of analyzing the emission spectrum of the dischargeto determine the composition and impurity content of the gas.
 19. Amethod of nondestructively analyzing the gaseous content of an enclosureat a pressure of at least 0.1 atm, said method comprising the steps of:generating an RF field with a helical coil resonator sufficient toeffect a discharge in the gas; contacting the helical coil resonator toa dielectric wall of the enclosure to thereby generate a discharge inthe gas contained therein; and spectrally analyzing the discharge. 20.The method of claim 19 wherein the step of generating an RF fieldincludes the step of applying RF power to the helical coil resonator sothat the resonator generates a voltage in excess of one hundred timesthe voltage of the RF power applied thereto.
 21. The method of claim 19wherein the step of generating an RF field includes the step of applyingRF power to the helical coil resonator so that the frequency of the RFpower matches the resonant frequency of the helical coil resonator. 22.The method of claim 19 wherein the step of generating an RF fieldincludes the step of applying RF power to the helical coil resonator sothat the impedance of the RF power matches the input impedance of thehelical coil resonator.
 23. The method of claim 19 wherein the step ofgenerating an RF field includes the step of applying RF power to thehelical coil resonator so that the length of the helical coil is aboutone quarter of the wavelength of the RF power applied thereto.
 24. Themethod of claim 19 wherein the step of spectrally analyzing thedischarge includes the step of determining the presence of selectedimpurities.
 25. The method of claim 24 wherein the selected impuritiesinclude one or more of oxygen, hydrogen, and carbon.
 26. The method ofclaim 24 wherein the impurity concentration by volume is less than about1 percent.
 27. The method of claim 26 wherein the impurity concentrationby volume is less than about 0.1 percent.
 28. The method of claim 27wherein the impurity concentration by volume is less than about 0.01percent.
 29. The method of claim 19 wherein the pressure of the gas isbetween about 0.1 atm and about 2.0 atm.
 30. The method of claim 29wherein the pressure of the gas is about 0.5 atm.
 31. The method ofclaim 19 wherein the enclosure forms the outer jacket of a highintensity discharge lamp.
 32. The method of claim 31 wherein the jacketcontains nitrogen at a pressure of about 0.5 atm.
 33. The method ofclaim 19 wherein the step of generating an RF field includes the step ofapplying RF power to the helical coil resonator at a frequency betweenabout 100 kHz and 100 MHz.
 34. The method of claim 33 wherein the stepof generating an RF field includes the step of applying RF power to thehelical coil resonator at a frequency of about 10 MHz.
 35. The method ofclaim 19 wherein the step of generating an RF field includes the step ofapplying pulsed RF power to the helical coil resonator.
 36. The methodof claim 35 wherein the RF power is pulsed at a frequency between about10 Hz and about 1000 Hz
 37. The method of claim 36 wherein the RF poweris pulsed at a frequency of about 120 Hz.
 38. The method of claim 35wherein the RF power has a duty cycle between about 5% and about 20%.39. The method of claim 38 wherein the RF power has a duty cycle ofabout 10%.
 40. A system for generating a discharge in a high pressuregas comprising: a dielectric enclosure containing high pressure gas; anRF generator for generating an RF field sufficient to effect a dischargein the high pressure gas, said generator comprising: an RF power source;and a helical coil resonator, said resonator being connected at one endto said RF power source and forming an electrode at the other endthereof, said electrode being in contact with said dielectric enclosureto thereby establish a discharge in the gas contained therein.
 41. Thesystem of claim 40 further comprising means for spectrally analyzing thedischarge generated in the high pressure gas.
 42. The system of claim 41wherein the dielectric enclosure comprises the outer jacket of an HIDlamp containing a fill gas at a pressure of about 0.5 atm.
 43. Thesystem of claim 42 wherein the system detects the presence of hydrogenin the jacket at concentrations by volume of at least 0.1%.
 44. Thesystem of claim 42 wherein the system detects the presence of oxygen inthe jacket at concentrations by volume of at least 0.3%.
 45. The systemof claim 40 wherein the frequency of the RF power source is matched withthe resonance frequency of the helical coil resonator.
 46. The system ofclaim 40 wherein the length of the helical coil in said helical coilresonator is about one fourth of the wavelength of the RF power providedby said RF power source.
 47. The system of claim 40 wherein the voltageat the electrode of said helical coil resonator is at least 50 timesgreater than the voltage of the RF power source.
 48. The system of claim47 wherein the voltage at the electrode of said helical coil resonatoris at least 100 times greater than the voltage of the RF power source.